MOS-technology power device manufacturing process

Abstract: A process for the manufacturing of a MOS-technology power device comprises the steps of: forming a conductive insulated gate layer (10) on a surface of a lightly doped semiconducter material (3) of a first conductivity type; selectively removing the insulated gate layer (10) from selected portions of the semiconductor material layer (3) surface; selectively implanting at a prescribed high energy a heavy dose of a first dopant of a second conductivity type into said selected portions of the semiconductor material layer (3), the insulated gate layer (10) acting as a mask, so that the peak concentration of the first dopant is located at a prescribed distance from the semiconductor material layer (3) surface; performing a thermal diffusion of the implanted first dopant with a prescribed thermal budget to from body regions (2) each comprising a first, heavily doped portion (5) substantially aligned with the edges of the insulated gate layer (10), and a second, lightly doped portion (6) formed by lateral diffusion of the first dopant under the insulated gate layer (10) and forming a channel region; selectively implanting a heavy dose of a second dopant of a first conductivity type into the first, heavily doped portions (5) of the body regions (2), to form source regions (7) substantially aligned with the edges of the insulated gate layer (10). The channel region (6) is produced by lateral diffusion from the heavily doped portion (5) of the body region. The device can be a power MOSFET or a IGBT.

Description: The present invention relates to a MOS-technology power device manufacturing process. MOS-technology power devices (such as power MOSFETs and IGBTs) are made up of a plurality of elementary functional units connected in parallel to conduct each one a respective fraction of the overall power device current. Each elementary functional unit comprises a so-called "body region" of one conductivity type (P type for an N-channel device), formed in a lightly doped semiconductor layer of an opposite conductivity type (N type) common to all the functional units; a heavily doped source region of said opposite conductivity type (N type) is provided inside the body region; the portion of the body region comprised between the source region and the edge of the body region itself is covered by a thin oxide Layer (called "gate oxide") and by a polysilicon layer ("gate layer"), and forms a channel region of the elementary functional unit. The source regions of all the elementary functional units are contacted by a same metal layer constituting a source electrode of the power device. Typically, the body regions have a square plan, and the power device is made up of a bidimensional array of square-plan elementary cells. Another conventional layout provides for hexagonal-plan elementary cells. It appears that to each elementary cell there is associated a parasitic vertical bipolar junction transistor (BJT) having emitter, base and collector respectively represented by the source region, the portion of the body region under the source region, and the lightly doped semiconductor layer (an NPN transistor in the case of an N-channel power device). For a correct working of the power device, it is extremely important to prevent such a parasitic transistor from being triggered on: in the case of a power MOSFET, the parasitic transistor can in fact Lower the breakdown voltage of the power device; in the case of an IGBT, two complementary parasitic bipolar transistors are present, forming a parasitic SCR which, if triggered on, can cause the destruction of the power device. A common technique used to prevent the triggering on of the parasitic bipolar transistor associated to the source and body regions provides for short-circuiting them in each elementary cell: the base-emitter junction of the parasitic transistor is thus short-circuited. In this condition the parasitic transistor breakdown voltage is BV CES (breakdown voltage between collector and emitter with the base short-circuited to the emitter). In the practice, the body region comprises a central heavily doped region (sometimes called "deep body region") and a lateral Lightly doped channel region; the source region has an annular shape and is formed in said lateral channel region, substantially around the deep body region. The source metal layer which contacts the source region also contacts the deep body region of all the elementary cells. A conventional manufacturing process provides for: epitaxially growing a lightly doped semiconductor Layer of a first, e.g. N, conductivity type over a heavily doped substrate of the N conductivity type, in the case of a power MOSFET, or of the P conductivity type, in the case of an IGBT; maskedly implanting and diffusing a heavy dose of a P type dopant into selected regions of the N type Layer, to form the deep body regions of the elementary cells; thermally growing a thin oxide Layer over the surface of the N type layer; depositing a polysilicon layer over the thin oxide Layer; selectively etching the polysilicon layer around the deep body regions; implanting a low dose of a P type dopant using the polysilicon layer as a mask; diffusing the P type dopant to form channel regions extending under the thin oxide layer; and maskedly implanting a heavy dose of an N type dopant into the deep body and channel regions of the cells to form annular source regions. The doping level of the deep body regions must be suitable to obtain low-resistance metal-semiconductor contact regions for the body regions, while the doping level of the channel regions is adjusted on the basis of the threshold voltage value desired for the power device; typical implantation doses are in the range of 10 13 -10 14 atoms/cm 2 for the channel regions, and 10 15 atoms/cm 2 for the deep body regions. Three distinct masks are necessary to introduce the dopants for the various regions of the elementary cells. The windows opened in the implantation mask for the deep body regions are smaller than the windows opened by etching in the polysilicon layer, to avoid that the lateral diffusion of the heavily doped deep body regions during the thermal steps alters the doping profile of the channel regions. The source regions are implanted in a self-aligned way with the edges of the windows in the polysilicon layer, but the implantation mask must further provide unexposed surface regions in the middle of the deep body regions. It appears that in the elementary cell the source region extends for a significant part in the lightly doped portion of the body region. Since the portion of the body region near the channel region under the source region has a relatively high sheet resistance (in the range of 600 ohm/square), the short-circuit of the base-emitter junction of the parasitic bipolar transistor becomes less effective as the lateral distance from the deep body region increases (a resistor is in fact introduced between the emitter and the base regions of the parasitic transistor, and its breakdown voltage lowers to BV CER , the breakdown voltage between collector and emitter with a resistor R connected between base and emitter). It would be therefore desirable to make the deep body region extend as far as possible under the source region. The problem is that, due to the lateral diffusion of the deep body region dopants during the thermal steps of the fabrication process, the doping profile in the channel region could be altered and the threshold voltage of the power device scarcely controlled. Thus, in current MOS-technology power devices, the region near the edge of the source region to the channel region of the elementary cells is critical from the point of view of the parasitic transistor triggering on. EP-A-0304 839 discloses a process as set forth in the preamble of claim 1. In view of the state of the art described, a main object of the present invention is to provide a manufacturing process for MOS-technology power devices suitable to obtain structures less affected by the abovementioned problems. According to the present invention, such object is attained by means of a process for the manufacturing of a MOS-technology power device as set forth in claim 1. Thanks to the manufacturing process according to the present invention, the source regions of the elementary cells are almost completely included in the heavily doped portion of the body regions, rather than in the lightly doped channel regions. The base series resistance of the parasitic vertical BJT having emitter and base respectively represented by the source region and the underlying portion of heavily doped body region of each cell is therefore negligibile; this characteristic is further enhanced by the fact that, by adjusting the implantation energy, the dopant concentration in the heavily doped portion of the body region can have a peak located under the source region, rather than at the surface of the semiconductor layer. The base series resistance of the parasitic BJT is thus further reduced, so that the base/emitter junction of the BJT is effectively short-circuited not only externally by the source metal electrode which contacts the surface of the source regions and of the heavily doped portion of the body regions of the cells, but also internally. The threshold voltage of the cells, i.e. the doping level of the channel regions, is adjusted by controlling the implantation dose and energy of the body region, and the temperature and length of the thermal diffusion process (thermal budget). It also appears that the process according to the invention allows a reduction in the mask number with respect to the conventional processes. The features of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is a cross-sectional view of a MOS-technology power device obtained by means of a process according to the present invention; Figures 2-6 are cross-sectional views taken at intermediate steps of said process; Figure 7 shows a dopant concentration profile along the direction of the z axis in Figure 4; Figure 8 shows a dopant concentration profile along the direction of the z axis in Figure 1; Figure 9 shows a dopant concentration profile along the direction of the x axis in Figure 1. In Figure 1 a cross-section of a MOS-technology power device is shown, more precisely a power MOSFET. The power MOSFET is made up of a plurality of elementary functional units 1 each comprising a P type body region 2 formed in a lightly doped N type ("N-") epitaxial layer 3 grown over a heavily doped N type ("N+") substrate 4. The elementary functional units are for example represented by square-plan elementary cells. The P type body region 2 of each elementary cell 1 comprises a first, heavily doped ("P + ") portion 5, and a second, more lightly doped ("P-") lateral portion 6 forming a channel region of the elementary cell 1. Each cell 1 also comprises an N + source region 7 formed in the heavily doped portion 5 of the body region 2 and having an annular shape, with a hole in the middle. An insulated gate layer, comprising a polysilicon gate layer 10 superimposed over a thin gate oxide layer 9, extends between adjacent elementary cells 1 and covers the surface of the channel region 6 of the body region 2. The heavily doped portion 5 of the body region 2 and the source region 7 are both substantially aligned with the edges of the insulated gate layer. The insulated gate layer is covered by an insulation layer 11, such as the so-called "PVapox", in which contact windows 12 are opened over the source region 7 and the heavily doped portion 5 of the body region 2, to allow a superimposed metal Layer 13 to contact them. The metal Layer 13 contacts in the same way all the elementary cells, and forms a source electrode S of the power MOSFET. A drain electrode D is also formed by covering the bottom surface of the substrate 4 by a metal layer 14. Figures 8 and 9 shows the dopant concentration profiles in the different cell regions: Na and Nd are the concentrations of acceptors (P type regions) and donor (N type regions) dopants, respectively. More precisely, Figure 8 represent an ideal section along the direction of the z axis in Figure 1, whereas Figure 9 is an ideal section along the x axis (i.e. along the channel of the cell). Since the source region 7 is almost completely internal to the heavily doped portion 5 of the body region 2, the base series resistance of the parasitic vertical BJT having emitter and base represented by the source region 7 and the underlying portion of the body region 2 is negligible even near the external edge of the source region. The base/emitter junction of such parasitic BJT is thus effectively short-circuited by the metal layer 13, to prevent it from being triggered on. The breakdown voltage of the power MOSFET is thus not reduced. A manufacturing process according to the present invention starts with the epitaxial growth of a lightly doped N type layer 3 over a heavily doped N type substrate 4. The surface of the N- layer 3 is then covered by a thin gate oxide layer 9 (e.g. thermally grown), over which a polysilicon gate layer 10 is formed (e.g. by deposition). In this way, an insulated gate layer is formed (Fig. 2). The polysilicon layer 10 is then selectively removed from the surface of the thin gate oxide layer 9. This step is performed in a per-se known way by covering the insulated gate layer with a photoresist 20, and removing it from the regions of the insulated gate layer surface which are to be exposed to the action of an etchant with high selectivity towards polysilicon with respect to the photoresist material (Fig. 3). Nothing changes if at this step the thin gate oxide layer 9 is selectively removed together with the plysilicon layer 10. A heavy dose of a P type dopant (such as boron) is then selectively implanted at high energy into the N- layer 2, the insulated gate layer (and, if necessary, the superimposed photoresist 20) acting as a mask for the implanted dopant ions. Due to the high implantation energy, P type regions 21 are obtained wherein the distribution of implanted ions has a peak located at a prescribed distance from the surface of the N-layer 2, depending on the implantation energy; as an example, the dopant ions can be implanted in a dose of 10 14 -10 15 atoms/cm 2 with an energy ranging from 100 to 200 KeV. Since moreover the insulated gate layer is used a mask, said P type regions 21 are laterally self-aligned with the edges of the insulated gate layer (Figs. 4 and 7). The implanted ions are then made to diffuse by means of a thermal process; the temperature and time duration (i.e. the so-called "thermal budget") of the thermal process determine the entity of the diffusion. The vertical diffusion of the dopant ions allows the formation of heavily doped P type regions 5, while the Lateral diffusion of the dopant ions forms Lightly doped channel regions 6 under the insulated gate layer (Fig. 5). With the dose and energy values previously cited, a suitable thermal budget is in the range 1050-1100°C for 0.5-2 hours. Channel lenghts of 0.5-1 µm can be obtained. A heavy dose of an N type dopant is then ion-implanted into the heavily doped regions 5; the implantion mask is provided in part by the insulated gate layer, but must further comprise photoresist isles 22 in the middle of the regions 5 to prevent the ions from being implanted in the central portions of the regions 5 which will be contacted by a metal layer (Fig. 6). After dopant diffusion, annular source regions 7, self-aligned with the insulated gate layer and substantially internal to the heavily doped regions 5, are formed. The portions of the thin gate oxide layer 9 not covered by the polysilicon layer 10 are then removed. An insulation layer 11 is formed over the insulated gate layer, and it is selectively removed to open contact windows 12 over the elementary cells 1. The insulation layer 11 is covered by a metal layer 13 which contacts the source region 7 and the heavily doped region 5 of all the elementary cells 1. A metal layer 14 is also formed on the bottom surface of the N + substrate 4. The process according to the present invention applies as well to the fabrication of IGBTs, the only difference being the conductivity type of the substrate. Further, the process according to the present invention is not limited to MOS-technology power devices made up of arrays of square-plan elementary cells as those previously described, but extends to high-density devices made up of a plurality of elementary functional units, wherein the body region is an elongated stripe with one dimension much greater than the other, and the source region comprises two regions elongated in the same direction as the body region, and a plurality of transversal regions. Such a device is described in the co-pending European Patent Application No. 94830288.0.

Claims: Process for the manufacturing of a MOS-technology power device, comprising the steps of: a) epitaxially growing a lightly doped semiconductor material layer (3) of a first conductivity type over a first surface of a heavily doped semiconductor substrate (4); b) forming a conductive insulated gate layer (10) on top surface of the lightly doped semiconductor material layer (3); c) selectively removing the insulated gate layer (10) from selected portions of the semiconductor material layer (3) top surface; d) selectively implanting in one step a first dopant of a second conductivity type into said selected portions of the semiconductor material layer (3), the insulated gate layer (10) acting as a mask; e) performing a thermal diffusion of the implanted first dopant to form body regions (2); f) masking portions of the surface of the body regions and implanting a heavy dose of a second dopant of a first conductivity type into the body regions (2) to selectively form inside the body regions annular source regions (7) having peripheral edges substantially aligned with the edges of the insulated gate layer (10); g) Forming a first electrode (13,S) contacting the source regions (7) and the body regions (2); h) disposing a second electrode (14,D) over a second surface of the semiconductor substrate (4), characterized in that in step d) the first dopant is implanted in a heavy dose and at a prescribed high energy so as to locate a peak dopant concentration of the first dopant under are as where the source region (7) will be formed, in step e) the thermal diffusion process involves a thermal budget suitable to form the body regions (2) with central portions (5) substantially aligned with the edges of the insulated gate layer (10) and heavily doped to make a base series resistance of a parasitic vertical BJT negligible, and lateral channel regions (6) extending under the insulated gate layer (10) and lightly doped to set a threshold voltage of the MOS technology power device, the source regions (7) being substantially internal to the heavily doped portions (5) of the body regions (2). Process according to claim 1, characterized in that said first dopant is implanted in a dose ranging from 10 14 to 10 15 atoms/cm 2 , and said prescribed thermal budget provides for submitting the power device to 1050-1150 °C for 0.5-2 hour. Process according to claim 1, characterized in that step 3) provides for: f) covering the insulated gate layer (10) and said selected portions of the semiconductor material layer (3) surface with an insulating material layer (11); g) opening contact windows (12) in the insulating material layer (11) over said selected portions of the semiconductor material Layer (3) surface; h) covering the insulating material layer (11) with a conductive material layer (13). Process according to claim 1, characterized in that said insulated gate layer (10) is formed by growing a thin oxide layer (9) over the surface of the semiconductor material Layer (3), and covering the thin oxide layer (9) with a further conductive material Layer (10). Process according to claim 4, characterized in that said further conductive material layer (10) comprises a polysilicon layer. Process according to claim 1, characterized in that said semiconductor material layer (3) is an epitaxial Layer grown over the heavily doped semiconductor material substrate (4). Process according to claim 6, characterized in that said semiconductor material substrate (4) is doped of the first conductivity type, so that the power device is a power MOSFET. Process according to claim 6, characterized in that said semiconductor material substrate (4) is doped of the second conductivity type, so that the power device is an IGBT. Process according to anyone of the preceding claims, characterized in that said first conductivity type is the N type, and said second conductivity type is the P type. Process according to anyone of claims 1 to 8, characterized in that said first conductivity type is the P type, and said second conductivity type is the N type. MOS-technology power device comprising a lightly doped semiconductor material layer (3) of a first conductivity type superimposed over a first surface of a heavily doped semiconductor substrate (4) and in which a plurality of elementary functional units (1) are formed, each comprising a body region (2) of a second conductivity type inside which a heavily doped source region (7) of the first conductivity type is provided, each body region (2) having a central portion (5) substantially aligned with edges of the insulated gate layer (10), heavily doped to make a base series resistance of a vertical parasitic BJT negligible a lateral portion (6) lightly doped to set a threshold voltage of the MOS-technology power device and covered by an insulated gate layer (10) to form a channel region of the elementary functional unit, a first electrode (13,S) insulatively disposed over the insulated gate layer (10) and contacting the source regions (7) and the body regions (2), and a second electrode (14,D) disposed over a second surface of the semiconductor substrate (4), said source region (7) being substantially internal to the central, heavily doped portion (5) of the body region (2), characterized in that the central portion (5) of the body region (2) has a peak dopant concentration located under the source region (7). MOS-technology power device according to claim 11, characterized in that said semiconductor substrate (4) is of the first conductivity type, and the power device is a power MOSFET. MOS-technology power device according to claim 11, characterized in that said semiconductor substrate (4) is of the second conductivity type, and the power device is an IGBT. MOS-technology power device according to anyone of claims 11 to 13, characterized in that said first conductivity type is the N type, and said second conductivity type is the P type. MOS-technology power device according to anyone of claims 11 to 13, characterized in that said first conductivity type is the P type, and said second conductivity type is the N type.

Inventors: FERLA GIUSEPPE, FRISINA FERRUCCIO, FERLA, GIUSEPPE, FRISINA, FERRUCCIO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L21
IPC codes: H01L29, H01L21